7.1 Atmospheric pressure: concepts and measurements

Atmospheric pressure, the force exerted by air molecules, plays a crucial role in weather patterns. It's measured using various instruments, from traditional mercury barometers to modern digital sensors. Understanding pressure helps us predict weather and analyze atmospheric conditions.

High and low pressure systems drive wind patterns and influence weather. Pressure gradients, the rate of pressure change over distance, are key to understanding atmospheric dynamics. Meteorologists use pressure data to create weather maps and forecast future conditions.

Atmospheric pressure and its significance

Defining atmospheric pressure

Top images from around the web for Defining atmospheric pressure

• 

  Pressure variation over altitude – TikZ.net View original

  Is this image relevant?

• 

  Atmospheric pressure - Wikipedia View original

  Is this image relevant?

• 

  LABORATORY 3: ATMOSPHERE COMPOSITION, PRESSURE, AND CIRCULATION – Physical Geography Lab Manual ... View original

  Is this image relevant?

• 

  Pressure variation over altitude – TikZ.net View original

  Is this image relevant?

• 

  Atmospheric pressure - Wikipedia View original

  Is this image relevant?

1 of 3

Top images from around the web for Defining atmospheric pressure

• 

  Pressure variation over altitude – TikZ.net View original

  Is this image relevant?

• 

  Atmospheric pressure - Wikipedia View original

  Is this image relevant?

• 

  LABORATORY 3: ATMOSPHERE COMPOSITION, PRESSURE, AND CIRCULATION – Physical Geography Lab Manual ... View original

  Is this image relevant?

• 

  Pressure variation over altitude – TikZ.net View original

  Is this image relevant?

• 

  Atmospheric pressure - Wikipedia View original

  Is this image relevant?

1 of 3

• Atmospheric pressure represents force exerted by weight of atmosphere per unit area at a given point on Earth's surface
• Measured in units of pascals (Pa), millibars (mb), or inches of mercury (inHg)
  • 1 atmosphere equals 101,325 Pa, 1013.25 mb, or 29.92 inHg at sea level
• Drives wind patterns and weather systems
  • Crucial parameter in meteorological forecasting and analysis
• Pressure gradients indicate rate of change of pressure over distance
  • Fundamental in understanding atmospheric dynamics and weather pattern formation

High and low pressure systems

• High-pressure systems associated with fair weather
  • Clockwise circulation in Northern Hemisphere
• Low-pressure systems often bring unsettled weather
  • Counterclockwise circulation in Northern Hemisphere
• Isobars visualize pressure distributions on weather maps
  • Essential tools for identifying weather systems
  • Lines of constant pressure
• Examples of pressure systems:
  • Hurricanes (low pressure)
  • Anticyclones (high pressure)

Instruments for measuring atmospheric pressure

Mercury-based instruments

• Mercury barometers balance atmospheric pressure against weight of mercury column in glass tube
  • Traditional and highly accurate method
• Barographs continuously record atmospheric pressure over time
  • Use aneroid barometer connected to pen
  • Traces pressure changes on rotating drum (paper chart)
• Fortin barometer allows adjustment of mercury level for precise measurements
  • Used as a reference standard in many weather stations

Modern electronic instruments

• Aneroid barometers use partially evacuated metal chamber
  • Expands or contracts with pressure changes
  • Connected to mechanical or electronic readout
• Digital barometers employ pressure sensors
  • Convert pressure changes into electrical signals
  • Often use piezoresistive or capacitive sensors
  • Provide precise digital display
• Examples of digital barometers:
  • Vaisala PTB330 (high-precision meteorological barometer)
  • Kestrel 5500 Weather Meter (portable multi-function device)

Atmospheric profiling instruments

• Radiosondes measure vertical pressure profiles in atmosphere
  • Weather balloons equipped with pressure sensors
  • Provide data up to stratosphere (30 km altitude)
• Aircraft-based sensors in AMDAR system
  • Measure pressure at various altitudes during flight
  • Contribute to global weather observation network
• Examples of radiosonde systems:
  • Vaisala RS41 (widely used modern radiosonde)
  • Lockheed Martin LMG6 (GPS-enabled radiosonde)

Interpreting atmospheric pressure data

Surface weather observations

• Surface weather station reports include pressure readings
  • Typically in millibars or inches of mercury
  • Often adjusted to sea level for consistency across stations
• Pressure tendency indicates change in pressure over time
  • Reported as rising, falling, or steady
  • Crucial for short-term weather forecasting
• Examples of pressure tendencies:
  • Rapidly falling pressure (approaching storm system)
  • Steadily rising pressure (improving weather conditions)

Synoptic and upper-air analysis

• Synoptic weather maps display isobars to show pressure patterns
  • Closely spaced isobars indicate strong pressure gradients and potential for high winds
• Upper-air charts present pressure data at various altitudes
  • Often use constant pressure surfaces (500 mb chart)
  • Analyze atmospheric flow patterns and jet streams
• Examples of upper-air chart types:
  • 850 mb chart (low-level moisture and temperature analysis)
  • 300 mb chart (jet stream analysis)

Advanced data sources

• Satellite-derived pressure data from microwave sounders
  • Provide global coverage of atmospheric pressure at different levels
  • Crucial for data-sparse regions (oceans, remote areas)
• Reanalysis datasets combine observations and model data
  • Create comprehensive, gridded pressure fields
  • Used for climate studies and historical weather analysis
• Examples of reanalysis datasets:
  • NCEP/NCAR Reanalysis
  • ERA5 (ECMWF Reanalysis v5)

Altitude vs atmospheric pressure

Pressure-altitude relationship

• Atmospheric pressure decreases with increasing altitude
  • Due to reduced weight of air column above
• Rate of pressure decrease with height follows non-linear relationship
  • Approximated by barometric formula or hydrostatic equation in meteorology
• Standard atmosphere model defines pressure values at different altitudes
  • 1013.25 mb at sea level
  • About 500 mb at 5,500 meters (18,000 feet) altitude

Pressure concepts in meteorology and aviation

• Pressure altitude used in aviation
  • Altitude in standard atmosphere with given pressure
  • May differ from true altitude due to non-standard conditions
• Geopotential height crucial in upper-air meteorology
  • Height of pressure surface above mean sea level
  • Used for analyzing atmospheric circulation patterns
• Scale height of atmosphere approximately 8.5 km
  • Altitude where pressure decreases to 1/e (about 37%) of sea-level value
  • Illustrates rapid decrease of pressure with height in lower atmosphere

Pressure adjustments and applications

• Pressure reduction to sea level standard practice in surface weather observations
  • Allows consistent comparison of pressure readings across different elevations
• Altimeter setting in aviation adjusts aircraft altimeter to local pressure conditions
  • Ensures accurate altitude readings for safe flight operations
• Examples of pressure-altitude relationships:
  • Denver, Colorado (5,280 feet elevation) typical surface pressure around 850 mb
  • Mount Everest summit (29,029 feet) pressure about 330 mb